feat(app): multicam panel with live angle grid, switching, timeline enable

- New MulticamPanel: rows/cols angle grid with the current angle
  highlighted, click-to-switch, 1-9 switch-and-split and cmd-1-9
  switch-only shortcuts (focused-panel routed), deferred switch queue
  during playback.
- src/oakui/multicam.rs: clip->connected-sequence resolution, multicam
  state detection (selection then playhead fallbacks), per-angle frame
  requests rendered through the process backend into an LRU cache.
- Timeline clip context menu Multi-Cam checkable item wired to
  oaktimeline::multicam enable/disable with undo.
- Engine trait extended (real + mock); mock drives the real command
  path with synthesized angle frames.

crates/oakengine.bk/include/timeline/displaymode.h

@@ -0,0 +1,51 @@
+/***
+
+  Oak Video Editor - Non-Linear Video Editor
+  Copyright (C) 2026 Oak Team
+
+  This program is free software: you can redistribute it and/or modify
+  it under the terms of the GNU General Public License as published by
+  the Free Software Foundation, either version 3 of the License, or
+  (at your option) any later version.
+
+  This program is distributed in the hope that it will be useful,
+  but WITHOUT ANY WARRANTY; without even the implied warranty of
+  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+  GNU General Public License for more details.
+
+  You should have received a copy of the GNU General Public License
+  along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+***/
+
+#ifndef OAK_EDITOR_TIMELINE_DISPLAYMODE_H
+#define OAK_EDITOR_TIMELINE_DISPLAYMODE_H
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/**
+ * @brief Shared timeline display-mode constants.
+ *
+ * Neutral home for the enum values behind the TimelineThumbnailMode /
+ * TimelineWaveformMode config keys; mirrors olive::Timeline::ThumbnailMode
+ * / WaveformMode (src/timeline/src/timelinecommon.h) and must stay
+ * value-compatible with them.
+ */
+enum OakTimelineThumbnailMode {
+	OAK_TIMELINE_THUMBNAIL_OFF = 0,
+	OAK_TIMELINE_THUMBNAIL_IN_OUT = 1,
+	OAK_TIMELINE_THUMBNAIL_ON = 2
+};
+
+enum OakTimelineWaveformMode {
+	OAK_TIMELINE_WAVEFORMS_DISABLED = 0,
+	OAK_TIMELINE_WAVEFORMS_ENABLED = 1
+};
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif // OAK_EDITOR_TIMELINE_DISPLAYMODE_H